1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to the bearings that support the main rotating component of the engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
There are two basic types of bearings used in a gas turbine engine: the ball bearing and the roller bearing. The main rotating component of a gas turbine, the compressor and turbine assembly, must be supported both axially and radially. When the direction of a load is at right angles to the shaft, it is called a radial load. When it is parallel to the shaft, it is called a thrust or axial load. Radial loads are due to rpm changes and aircraft maneuvering, while axial loads result from thrust loads (forward and rearward) from the compressor and turbine. A ball bearing will limit or support both radial and axial loads, while a roller bearing will limit or support only radial loads. Since there is always engine growth because of temperature changes in the engine, one bearing supporting the compressor must always be a ball bearing to absorb both radial and axial loads, while the other must always be a roller bearing to allow axial movement due to changing dimensions in the engine. This is always true for the turbine rotor in larger engines. FIG. 1 shows this prior art arrangement with a ball bearing supporting the front or compressor end of the rotor shaft and a roller bearing supporting the rearward or turbine end of the rotor shaft.
The ball bearing is used in the front end of the engine due to the cooler operating environment. The roller bearing is used in the turbine end because the roller bearing can allow for a small relative axial displacement of the races with respect to the roller elements. Also, the ball bearing does not require a cage. The roller bearing requires a cage and therefore generates more heat from the rubbing of the rollers against the cage. In a small gas turbine engine in which the rotational speeds of the rotor shaft are very high, the cage will develop excessive heat due to the rubbing. Also, the applicant has discovered that the rollers tend to turn sideways when the outer race is displaced axially with respect to the inner race due to the axial loads placed on the bearing. When the roller elements turn sideways they start to skid and not roll. Because the roller bearing is not symmetrical (as in the ball bearing), thus sideways turning of the rolling element results whenever the axial load is applied to the bearing. This is a very unstable result for the bearing.